If, for example:
- we have a BJT buffer (power amplifier of 200 watts max, with no voltage gain) with approx 10KOhm input impedance (in the schematic attached to the first post we drew it with a simple resistor)
- I want to drive this buffer stage with the gomez drew as above,
- I want an hi control also on low frequencies

using a coupling capacitor of 1-2-3... or 10 uF (max) causes a very high output impedence of the Voltage stage, exactly where and when instead I need big "control".

The possible solutions:

- can I use a coupling capacitor of around 100 uF to have same impedance at 20 Hz?

- Don't you think using this big capacitor will worsen the sound, especially on mid-hi frequencies?

What kind of solution would you recommend (or use) in situation like this?

I think you are confusing pre-amplifier source impedance with power amplifier load (speaker) control or damping factor. Your amplifier exerts control over the speaker you are driving independent of the input source Z, it doesn't care what the source impedance is at its input terminals within reason as long as it is reasonably constant and a small fraction of the input impedance. According to a quick calculation you would be less than -0.5dB down @ 20Hz with a 2.2uF, note that large value high voltage film caps are very expensive, so this might be an ok compromise, certainly 4.7uF would be better, rolloff would be less than 0.2dB at 20Hz in this case. I would not go over 10uF in any case.

Another thing to consider is that larger film caps do not sound as good as smaller ones due to increased inductance, DA, and other variables and should probably also be bypassed by a much smaller higher quality film cap.

Something else to mention is that if you are using a solid state amplifier you must wait to turn it on until no more charging current is flowing in that large output coupling cap. I recommend a muting relay to short the output side of that cap to ground for at least 30 seconds after the pre-amp filaments and supply are FULLY warmed up in order to reduce dc at the input of your amplifier to 0. Failure to do this may result in blown output devices in your amplifier (possibly blown input transistors or ics as well) and fried voice coils in your woofers..

edit: add a thought or two

__________________"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

My doubts come from the simulation in "gomez.gif" (1st post), when you can see the output impedance increase a lot at lower frequencies.

And this is also when the bjt stage input impedance remain the same (let say 10 Kohm).

So it is true the low bandwith is -0.5 dB at 20 Hz, with a coupling cap of 2,2 uF, but it is also true that if the driving impedence of the BJT stage increase at 1Kohm or more, It will lack especially the bass controls.

We have the same effect if we use a Tube voltage stage with 2 Kohm output impedence (like 6Sn7 SRPP) or the same stage with, for example 6H30 (300 Ohm output impedence).
I want to say that is clearly listened the differencens IN LOW FREQUENCY CONTROLS between the two stages (I mean better 6H30 stage) ALTHOUGH both stage have 2,2 uF coupling cap and -0.5 dB at 20 Hz!

** So it is true the low bandwith is -0.5 dB at 20 Hz, with a coupling cap of 2,2 uF, but it is also true that if the driving impedence of the BJT stage increase at 1Kohm or more, It will lack especially the bass controls. **

Sorry, maybe it is not enough clear: I want to say:

It is true that the low bandwith is -0.5 dB at 20 Hz, with a coupling cap of 2,2 uF, but it is also true that

if the driving (VAS) impedence that the BJT stage sees, for istance, at 40 Hz, has increased at 1Kohm (or more... see the graph above), it will be (IT IS!!) very audible the consequent lack in the bass.

Well that is the implication of the falling response below (or sometimes even above ) the corner frequency, however it is not a great idea to use very large coupling caps with tube pre-amplifiers operating into low input impedance solid state amplifiers - this is a recipe for disaster. I did mention the turn on issue and totally forgot to mention the turn off issue which is of the same magnitude but opposite sign, either way it won't matter much if large amounts of dc current flow through your speaker voice coils.

Since you are so (rightly) concerned about the driving point impedance I would recommend you consider a really good line output transformer for your application - suitably sized & designed you will get great bass response and a very uniform source impedance across the entire audio spectrum. You can easily get source impedances of a few hundred ohms or less, particularly if you use something like 6H30 to drive it.
Also no dc issue to be concerned about either.

If you were to use a CCS or choke and parafeed configuration there are many off the shelf transformers you can use.

Bartolucci should be able to help you out with this. (Both parafeed and conventional SE line stage transformers.)

__________________"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine